T. Messelis, S. Haspeslagh, P. De Causmaecker B. Bilgin, G. Vanden Berghe
Introduction Method Our work Conclusions Future work T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 2
predict performance ◦ of one or more algorithms ◦ on a specific problem instance to be able to ◦ know in advance how good an algorithm will do ◦ choose the ‘best’ algorithm out of a portfolio ◦ choose the ‘best’ parameter setting T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 3
Build empirical hardness models ◦ empirical: performance of some algorithm ◦ hardness: measured by some performance criteria time spent by an algorithm searching for a solution quality of an (optimal) solution gap between found and optimal solution Model hardness as a function of features ◦ computationally inexpensive ‘properties’ e.g. clauses-to-variables ratio (SAT) e.g. maximum consecutive working days (NRP) T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 4
Introduced by K. Leyton-Brown et al. 1.Select problem instance distribution 2.Select one or more algorithms 3.Create a set of features 4.Generate an instance set, calculate features and determine the algorithm performances 5.Eliminate redundant or uninformative features 6.Use machine learning techniques to select functions of the features that approximate the algorithm’s performances K. Leyton-Brown, E. Nudelman, Y. Shoham. Learning the empirical hardness of optimisation problems: The case of combinatorial auctions. In LNCS, 2002 T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 5
This strategy has been successful in different areas: ◦ combinatorial auction: winner determination problem ◦ uniform random 3-SAT accurate algorithm performance prediction algorithm portfolio approach (SATzilla) won several gold medals in SAT competitions Apply it to Nurse Rostering! T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 6
problem of assigning nurses to shifts, given a set of hard and soft constraints Performance: ◦ time spent by a complete search algorithm to find the optimal roster ◦ quality of this optimal roster ◦ quality of a roster obtained by a heuristic algorithm, ran for some fixed period of time ◦ quality gap between both solutions T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 7
Translate NRP instances to SAT instances and use existing SAT features to build models translation based on numberings solve instances to optimum using CPLEX run a metaheuristic for 10 seconds T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 8
Regression results on predicting CPLEX objective T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 9 Regression Statistics Multiple R0, R Square0, Adjusted R Square0, Standard Error3, Observations500 ANOVAdfSSMSFSignificance F Regression , , , Residual , , Total CoefficientsStandard Errort StatP-valueLower 95%Upper 95% Intercept VCG CN variation549, , , ,6305E-69497, , VCG CN max16, , , ,1547E-12015, , VCG VN min-27, , , ,7809E , , CG mean-126, , , ,6608E , , BAL PL/C variation14819, , , ,9828E , ,28757 BAL , , , ,8022E , ,44717 BAL , , , ,3813E , ,843571
Come up with a feature set specifically for NRP and build models from these features on the same set of NRP instances for the same performance indicators T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 10
very simple set of features: ◦ some problem parameters max & min number of assignments max & min number of consecutive working days max & min number of consecutive free days ◦ and ratio’s of those parameters max cons working days / min cons working days max num assignments / min cons working days availability / coverage requirements (tightness) ... T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 11
Regression results on predicting CPLEX objective T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 12 Regression Statistics Multiple R0, R Square0, Adjusted R Square0, Standard Error2, Observations500 ANOVAdfSSMSFSignificance F Regression752571, , , ,2709E-297 Residual , , Total ,55 CoefficientsStandard Errort StatP-valueLower 95%Upper 95% Intercept66, , , ,6721E-16763, , max num assignments (6 - 10) - 6, , , ,9072E , , min cons working days (2 - 6)2, , , ,86075E-262, , max cons working days (3 - 8) - 2, , , ,2938E , , min cons free days (1 - 3)7, , , ,09346E-296, , max cons free days (2 - 3) - 2, , , , , , max num assignments / min cons work days1, , , ,24188E-060, , max cons FD / min cons FD2, , , ,0444E-051, ,
We can build accurate models to predict algorithm performance, based on very basic properties of NRP instances. ◦ objective values models for the objective values of both CPLEX and the metaheuristic are fairly accurate ◦ gap less accurate predictions, however with a standard error of 0.95 ◦ CPLEX running time not very accurate, due to very high variability in the running time T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 13
building models on a larger scale ◦ now only a very limited dataset more (sophisticated) features for NRP instances ◦ now only a very basic set with some aggregate functions of it combining both SAT features and NRP features T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 14
T. Messelis, S. Haspeslagh, P. De Causmaecker, B. Bilgin, G. Vanden Berghe 15